The phenomenon of the “buckling” of the rotor and of the casing of turbomachines such as gas turbines and steam turbines is sufficiently known. It is caused by the large and high-mass structures of such machines having stored large quantities of heat after prolonged operation. During the cooling, a pronounced vertical thermal stratification occurs in the comparatively large flow passages, this thermal stratification leading to nonuniform temperature distributions in the static and the rotating components, which, on account of the different thermal expansions, results in a distortion of casing and rotor and in deviations from the rotationally symmetrical desired geometry. As a result, with the unavoidably small gap dimensions in modern turbomachines, jamming of the rotor in the casing occurs, which inhibits the start-up availability and in addition is capable of putting the mechanical integrity at risk. Shaft-turning systems or also “shaft-indexing” systems have therefore been disclosed, for example, by U.S. Pat. No. 3,793,905 or U.S. Pat. No. 4,854,120. In this case, the rotor of a turbomachine continues to be rotated at a certain speed after the shutdown. Here, as in the known shaft indexing, low speeds within the region of 1 rev/min and lower are preferred. On the one hand, this is sufficient in order to make the cooling of the rotor more uniform in the circumferential direction; on the other hand, the speed is low enough in order not to cause any pronounced axial flow through, for instance, the hot-gas path of a gas turbine with associated cold-air input and thermal shocks.
In the section subjected to high temperatures, modern gas turbines are often constructed with twin-shell casings. In this case, an annular space, to which cooling air or other coolant is often admitted during operation, is formed between an inner casing and an outer casing. A vertical thermal stratification which leads to distortion of the casings forms in the annular space without further measures after the gas turbine has been shut down.
DE 507 129 and WO 00/11324 propose to provide means in a twin-shell casing of a turbomachine in order to disturb the stable thermal stratification when the turbomachine is at rest by a forced flow inside the intermediate space. In this case, it is essentially proposed to deliver medium outside the annular space from one point of the annular space to another point of the annular space, as a result of which a compensating flow is induced inside the annular space. The system proposed there, however, is an active system which has to rely on a blower for its functioning. However, a system which is intended to avoid the buckling of the casing and thus grazing of the blades of a turbomachine must be regarded as a safety system, which should work as passively as possible without having to rely on the functioning of active components.
DE 367 109 discloses a passive system for the flow of cooling air through components of an internal combustion engine. However, this is designed for convection optimization and is therefore already not suitable for fulfilling the present task, since importance is attached to convection optimization, a factor which in the present case entails undesirable thermal shocks. EP 0 014 941 specifies a system for passive ventilation in an intermediate space formed between two casing shells. To avoid thermal shocks, however, no pronounced flow through the casing cavity is to be produced in this case, but rather the formation of pronounced vertical thermal stratifications is merely to be avoided. Furthermore, the systems presented in DE 367 109 and EP 0 014 941 have no means of eliminating disturbing effects on the requisite flows which are present in the turbomachine casing cavities considered and are optimized in a complicated manner.